Due to the controllable reactivity made possible by ionized gas species, plasma processing has become an important technology associated with various production processes, including surface preparation, etching and deposition, the latter now often called plasma-enhanced chemical-vapor deposition (PECVD). The workpiece upon which these processes are performed is called the substrate.
Plasma sources utilized for such processes operate at both high pressures, generally in excess of 100 Torr and at low pressures, which are, for example, below 100 Torr. Low-pressure plasmas may be operated with direct current (DC) or alternating current (AC) power sources. The AC power sources are generally of the radio-frequency (RF) type, which operate generally at 100 MHz or below, or of the microwave variety, wherein the source operates typically at frequencies in excess of 100 MHz.
Most microwave plasmas in use for the purposes set forth above are called electron-cyclotron-resonance (ECR) plasmas, characterized in that a static magnetic field of sufficient strength results in efficient plasma production under the proper circumstances. The magnitude of the required magnetic flux density B is given by the relation B=(2 .pi.mf)/e where m and e are the mass and charge of the electron, respectively, and f is the frequency of the energy being carried by the waveguide. Solving with the constants yields f=0.0028B, where B is the magnetic flux density in Gauss, and f is the microwave frequency expressed in GHz. Thus, for example, for a frequency of 2.45 GHz, a frequency made popular by a large number of commercially available sources, a magnetic flux density of 875 Gauss is required in order to satisfy the resonance condition for efficient plasma production. (Frequencies of approximately 400 MHz and 915 are also important due to the availability of commercial sources.)
In prior-art devices, the required magnetic field is produced by solenoidal electric coils, permanent magnets, or by some combination of the two. The microwaves are then coupled to the vacuum chamber and the plasma therein by way of either a tunable resonant cavity or through a waveguide window composed of a dielectric material, this window incorporating a vacuum seal to ensure that the vacuum chamber remains isolated from atmospheric pressures and undesirable contaminants. The resonant-cavity approach ordinarily uses permanent magnets commonly arranged in a circle around an evacuated dielectric chamber, obviating the need for solenoidal electric coils in order to satisfy the resonance condition. In this case, the magnetic field strength associated with a particular permanent magnet falls off rapidly as a function of the distance from the magnet, so that most of the plasma is field-free.
The window approach utilizes a waveguide that delivers microwave power up to the window, where it may pass into the vacuum chamber and generate a plasma therein. The advantage of waveguide coupling is that a resonant cavity is not required. As a consequence, the diameters of the plasma and the chamber are not restricted by the need to satisfy resonant-cavity geometrical constraints imposed by the resonant cavity condition. Larger diameter plasmas are, therefore, made possible for a given microwave frequency.
Although waveguide coupling forgoes the requirement of a resonant cavity, and although it is less susceptible to geometrical constraints, currently available systems have drawbacks. The electromagnets present in waveguide-type plasma sources are typically configured to immerse the entire plasma in a magnetic field, requiring a strength ranging from 875 Gauss near the window to a few hundred Gauss near the substrate for a frequency of 2.45 GHz. These electromagnets are bulky and expensive to produce and operate. As a consequence, although waveguide-type microwave plasma sources appear to be among the most versatile and scalable to larger substrates and more demanding applications, there remains a need for a more efficient and elegant means for producing the magnetic field necessary to satisfy the resonance condition.